Interactions of positively identified human natural killer (NK) cells with appropriate target cells were studied in serial thin sections with the electron microscope. These studies were extended to human effectors of ADCC rat NK cells and rat tumor cells with NK and ADCC activities. All these cells contain prominent cytoplasmic granules and in all cases the granule contents are secreted by exocytosis under conditions which activate the effectors to kill. The material secreted by all effector types is heterogeneous and has as a major component membrane vesicles which bear ring-shaped profiles on their surfaces. Granules have been isolated from cells of several rat leukemias which have NK and ADCC activities. The isolated granule material is cytotoxic to a variety of target cells. Ultrastructural studies of the isolated granules show that membrane vesicles bearing ring shaped profiles are elaborated from the granules during the time and under the conditions in which the granule material is cytotoxic. Ultrastructural studies on cloned T cells of known functions have been continued. The morphology of inclusions in NK and suppressor clones have some similarities, while in proliferative or helper clones the inclusions are quite different.